There are various measuring instruments which are equipped with a replaceable probe system. The replaceable probe systems are to be removable as rapidly and uncomplicatedly as possible.
One embodiment of a conventional measuring instrument has a probe system which may be connected by means of spring force to a stylus instrument. This connection is produced by hooking on a hook system. An active mechanism having a servomotor is provided in order to perform the placement and removal of the probe system.
There are also examples of probe systems which are attached to a stylus instrument of a measuring instrument using permanent magnets. In case of a collision of the probe system with a workpiece to be measured, for example, the probe system falls down in order to thus avoid damage to the stylus instrument. It must be put on again manually. A corresponding example is described in German Utility Model DE-GM 7400071.
A further measuring instrument is disclosed in U.S. Pat. Specification No. 5,755,038. The measuring instrument is distinguished in that it provides a probe system which is connected via permanent magnets to a stylus instrument. There is a small air gap between the magnets. The probe system may be separated from the stylus instrument by applying a force. No spring is employed. In order to be able to ensure a secure hold of the probe system, the magnets must ensure a specific minimum attractive force. However, this has the disadvantage that when the probe system is put on, it is attracted abruptly. In addition, a relatively large force must be applied for the separation. Therefore, removal of the probe system is relatively difficult.
A similar approach is described in PCT Application WO 03/087708. In this case, in addition to the permanent magnets which hold together the probe system and the stylus instrument, a damper is employed, which is seated either on the side of the probe system or on the side of the stylus instrument. The damper is designed in such a way that as the probe system approaches the stylus instrument, a hard collision is prevented by the damper effect, which deploys first.
An approach with magnetically coupled working modules is described in the POT application with publication number WO 03/083407 A. According to this document a system for reducing the clamping force is arranged externally to a changing station. The set up is so that the strong magnetic attractive forces have to be overcome only during an automatic changing process when removing the working module. When attaching the working module, the full magnetic force is acting and a damping occurs only if an automated changing process takes place in the external changing station
An approach with magnetically coupled changing plate is described in the U.S. Pat. 6,012,230. A system for reducing the magnetic clamping force is situated in the changing plate. A reduction of the clamping force when removing the changing plate is ensured in connection with an automated changing process. In case of a manual changing process, the changing plate has to be grabbed in a specific manner and pulled, in order to actuate the mechanism for reducing the clamping force. When manually attaching it, the full magnetic force is applied too.
A further example of a measuring instrument having replaceable probe systems may be inferred from German Patent Specification DE 3320127 C2. A measuring instrument is described therein, whose probe system is attached using a combination of a permanent magnet and an electromagnet. The field of the electromagnet is superimposed on the field of the permanent magnet. An interchangeable plate having probe systems is pulled against a stylus instrument via a spring. The magnetic force of the permanent magnet may be briefly amplified by the electromagnet, or reduced by polarity reversal. When the interchangeable plate is put on the stylus instrument, there is an air gap between the permanent magnet and the interchangeable plate, due to which the magnetic attractive force is precisely large enough that the plate is attracted slightly by the stylus instrument. A spring is provided which holds back the permanent magnet. The magnetic force of the permanent magnet is increased in such a way by a current pulse that the spring force is overcome and the permanent magnet approaches the plate and finally presses against it. The plate is now pulled to the stylus instrument via the spring and there is no longer an air gap between the permanent magnet and the plate. To remove the plate, a current pulse is applied in the reverse direction, which reduces the magnetic force. The spring is now capable of pulling the permanent magnet away from the plate. An air gap thus arises and the plate may be removed—without having to apply large forces.
It is a disadvantage of the system that it has relatively large moving masses because of the electromagnet. In addition, cable connections to the mobile part of the stylus instrument are necessary in order to supply the coil of the electromagnet with power. A further disadvantage is that changing is only possible in the energized state. If a collision of the probe system occurs, relatively high forces arise on the stylus instrument, since the plate is held with the probe system on the stylus instrument by the spring force.
It is an object of the present invention to provide a device having a removable probe system, which allows automatic removal of the probe system to be performed. A further object of the present invention is to allow the simplest possible manual replacement of the probe system.
It is a further object to design a measuring instrument having a device having removable probe systems in such a way that a new calibration is not necessary after each replacement. In addition, damage to the stylus instrument is to be prevented.
In particular, the present invention is to achieve the object of improving a numerically controlled measuring instrument (CNC measuring instrument) in such a way that it has a simple and operationally reliable construction.